1. Field of the Invention
The invention relates to surface-modified nanostructured metal oxides which can be used in use as reinforcing fillers or as rheological additives for adjusting viscosity, yield point, shear-thinning, and also thixotropic properties in coating materials such as paints and varnishes, but also adhesives, sealants and plastics.
2. Description of the Related Art
Organofunctional particulate metal oxides are often used as active fillers for improving the mechanical properties of materials which have diverse uses. It is possible by this means, for example, to improve the scratch resistance of coatings such as paints and varnishes, or to vary the mechanical properties of adhesives and sealants or plastics, for example, silicone elastomers, in a targeted manner.
Moreover, even in the uncrosslinked state of the matrix systems, the nanostructured fillers perform an extremely important function. For example, the dispersions generally have relatively high viscosities and in many cases even viscoelastic properties. This pseudoplastic behaviour plays a decisive role in particular for the processing properties of the corresponding material. Through modification of the surface of the particulate fillers, it is possible to control the interactions with the matrix surrounding them and thus the viscoelastic behavior of the dispersion.
For the surface modification, compounds are often used which form chemically bonded, oligomeric dimethylsiloxane units under suitable process conditions. As described in the patent specification DE 1 163 784, these units can be built up from dimethylsiloxy units capable of condensation.
However, oligo- or polydimethylsiloxanes that are cyclic (DE 1 916 360) or linear (EP 0 686 676) are also used for the hydrophobicization. In the production processes applied therein, a process step is usually included in which drastic temperaturessignificantly higher than 250° C. prevail. In this way, it is possible to achieve a comparatively uniform modification of the surface. However, it is also generally known that polydimethylsiloxanes enter into depolymerization reactions under these temperatures, for which reason the resulting products are characterized by comparatively short dimethylsiloxane chains.
Last but not least, as high as possible a thickening effect of the metal oxide is generally desired for reasons of cost efficiency in the event of use as a rheological additive. When used in polar organic systems such as e.g. in epoxide or unsaturated polyester resins, the thickening effect of the resulting products largely correlates with the degree of coating. Thus, for example, the specifications DE 1 916 360 and EP 0 686 676 point to the importance of the high degree of hydrophobicization and thus low residual content of silanol groups on the oxide surface. However, the increasing incompatibility of the metal oxide with the polar target matrix mostly also leads to significantly increased incorporation times and/or poorer dispersion of the filler in the matrix.
Moreover, the surface modification of all of the dimethylsiloxy-functional metal oxides described in the aforementioned specifications are not sufficiently stable under alkaline conditions. Laboratory experiments relating to storage stability of dispersions of the metal oxides in amine-group-rich matrices mostly reveal a significant change in the pseudoplastic behavior over several hours or days.
Thus, the dimethylsiloxane-modified silicas available hitherto, for example when used as a thickener of a typical amine curing agent of a two-component epoxy adhesive, reveal no significant technical advantage compared to a nonmodified, hydrophilic silica. Thus, for example, the viscosity and thickening effect of HDK® H18 (surface-modified silica) is slightly higher compared to HDK® N20 (unmodified silica), but this has not led to a more widespread application of HDK® H18 since the costs of this silica are significantly higher.
If the currently available dimethylsiloxane-modified silicas are used in a typical STPE sealant formulations, then the compositions exhibit an increase in viscosity and a particularly greatly pronounced increase in shear stress. The latter has proven to be extremely disadvantageousduring the application of the compositions since considerably greater forces are required in order to overcome the yield point.